Why a black hole’s repeat flares keep fading: the answer was the star’s spin all along

When a star strays close to a supermassive black hole but isn’t destroyed outright, it can survive to make repeated passes, flaring each time it swings by. Astronomers have long been puzzled that in several of these systems the successive flares grow steadily dimmer — a pattern theoretical models couldn’t reproduce for years. A team at Syracuse University now shows the missing piece was a factor everyone had overlooked: how fast the star was already spinning. The study, led by doctoral student Ananya Bandopadhyay, was published in The Astrophysical Journal.

Stars that live to flare again

In a classic tidal disruption event, the difference in a black hole’s gravitational pull across a nearby star tears the star completely apart. The shredded debris falls inward, releasing energy as light over days to months — a brief burst of fuel that illuminates an otherwise invisible black hole and lets astronomers study it indirectly.

But if a star doesn’t come quite close enough to be fully destroyed, it loses only a fraction of its mass in a partial disruption. In a repeating partial event, the surviving core keeps orbiting and sheds more material with each close pass, months to years apart. Wide-field time-domain surveys, which repeatedly scan the sky for objects that change in brightness, let astronomers watch the same star-and-black-hole encounter play out over and over.

A two-year puzzle

Of the roughly ten repeating systems identified so far, four produce flares that grow progressively fainter. Decreasing mass loss seems like the obvious culprit — but earlier hydrodynamical simulations delivered a surprise: even as a star shed less material with each pass, the predicted flares stayed roughly the same brightness. As Bandopadhyay puts it, the team was puzzled by this for two years.

The reason the brightness held steady traced to a subtle effect. The black hole’s tides not only strip material, they also torque the star, spinning it up with each encounter. So although less material falls back, it returns over a shorter time, keeping the flare’s peak brightness roughly constant. To reproduce the dimming actually seen in the sky, the researchers needed a new ingredient — a star that was already spinning rapidly before its very first encounter.

That initial rotation, they found, prevents the star from being spun up much further during each pass. Without the extra spin-up, the timescale over which stripped material falls back stays relatively constant, so as the star loses less each time, the peak fallback rate — and thus the flare’s brightness — can finally decline. Mystery solved.

Following the clue back to a shredded binary

That raised a deeper question: why would the star already be spinning so fast? And, separately, how does a star end up bound so tightly to a supermassive black hole that it orbits in mere months? The team argues a single phenomenon answers both — the Hills mechanism.

In that scenario, a pair of closely orbiting stars passes near the black hole, which rips the binary apart, ejecting one star and capturing the other on a tight orbit. In a close binary, the two stars can become tidally locked, each spinning on its axis as fast as the pair orbits one another. The tighter the binary, the shorter that period — and the faster a tidally locked star spins. The binaries able to leave a captured star on the short orbits seen in these repeating events would have to be extremely tight, which also leaves that captured star already spinning rapidly. Both peculiarities, the fast spin and the tight orbit, fall out of the same event: the tidal destruction of a binary and the capture of one of its stars.

The implication reaches into our own galaxy. Hills capture may also have delivered some of the stars now orbiting Sagittarius A*, the Milky Way’s central supermassive black hole — meaning these findings could help explain the properties of stars in, as the team describes it, our own cosmological backyard.

© 2026 Homer Dávila Gutiérrez, FRAS — SKYCR.ORG. All rights reserved. Publication: Ananya Bandopadhyay et al., The Role of Stellar Spin in Repeating Partial Tidal Disruption Events, The Astrophysical Journal (2026). DOI: 10.3847/1538-4357/ae8f31